How do I connect the exposed pad for WSON packages used with Serial NOR devices?
The exposed pad does not have any electrical significance, and is not internally connected to the die. There is nothing electrically bonded to the pad on the bottom of the WSON package. Heat dissipation is also not a concern, even though the datasheet describes this as a heat sink slug. It is used to improve board-level reliability by attaching the pad to the board with solder. It can be connected to ground or left unconnected.
Here is a snapshot of the mechanical drawing from the FL-S datasheet (001-98283 Rev M) with the area in question highlighted in red: